Cellular plastics have been available for many years. One of the first of such materials was cellular rubber dating to the 1910-1920 period. Subsequently cellular compositions were made from latex, phenol-formaldehyde resins, urea-formaldehyde resins, PVC, polyurethane, cellulose acetate, polystyrene, polyethylene, epoxides, ABS resins, silicones and very recently polyphosphazenes. Polyphosphazene foams have very desirable properties in that they are highly fire resistant and when subject to direct flame do not produce toxic smoke which is encountered with many other common foamed materials, noteably polyurethanes.
Polyphosphazenes are polymers containing a plurality of --P&lt;.dbd.N-- groups wherein substituents are bonded to phosphorus. The polyphosphazenes which are the concern of this invention are high molecular weight linear polyphosphazenes containing 50 or more of the above units and having molecular weights from about 10,000 up to 5,000,000 or higher. They are substantially linear and have little if any cross-linking. In general, they are soluble in benzene, toluene, cyclohexane, and tetrahydrofuran and are relatively insoluble in linear aliphatic hydrocarbons such as hexane or heptane. Groups substituted on phosphorus include phenoxy, alkylphenoxy, alkoxyphenoxy, aminoalkylphenoxy, alkylaminoalkylphenoxy, dialkylaminoalkylphenoxy, halophenoxy (e.g., para-chlorophenoxy, meta-bromophenoxy, trifluorophenoxy and the like), haloalkylphenoxy (e.g., trifluoromethylphenoxy), alkoxy, haloalkoxy (e.g., trifluoroethoxy), alkenylphenoxy (e.g., ortho-allylphenoxy and the like).
Methods of making cellular polyphosphazenes are known. Various procedures are described in U.S. Pat. Nos. 4,026,838; 4,055,520; 4,055,523; 4,107,108; 4,189,413 and others. In general, the foams are made by mixing the polyphosphazene gum, a blowing agent and a peroxide or sulfur-type curing agent and heating the blended components to activate the blowing agent and cure the resultant foam. Control of the process to obtain uniform results leaves much to be desired. When the process is conducted in what appears to be the same manner based on mixing time, composition, foaming temperature and curing temperature, different results frequently occur. After making an acceptable foamed product, the next run although conducted with the same composition can give a poor result such as a split in the foam interior.